Massive Multiple-Input Multiple-Output Multi-User beamforming (MM-MUBF) offers the potential to significantly increase the spectral efficiency and throughput by many folds through spatial multiplexing, providing linear capacity growth without the need of increasing spectral bandwidth. However, when the number of RF chains and antennas becomes large (It is understood that an antenna is associated with a RF chain, transmitting (Tx) or receiving (Rx), thus, hereafter when the number of antennas is used, it should be understood to mean the number of antennas and the associated RF chains), there is significant overhead in channel estimation to obtain the CSI. For a Base Station (BS) with a large number of antennas, e.g., N antennas, to simultaneously Beam Form (BF) to multiple receivers, e.g., K User Equipment (UEs) and/or Small Cells (SCs) which depend on a BS to provide wireless backhaul, the BS transmitters must know the CSI of the NxK channels, where N>>K. To be precise, it is the CSI between N BS antennas and the total number of antennas on the K UEs and/or SCs. To simplify discussion, without loss of generality, we assume the total number of receiving antennas is K.
For this reason, prior art on massive MIMO systems focused on the Time-Division Duplexing (TDD) mode [1] because the transmitter can get the CSI of the receiver using channel reciprocity, which allows a BS to estimate its downlink (DL) channels from uplink (UL) pilots sent by the receivers, i.e., UEs and/or SCs. The overhead for estimating the DL CSI increases linearly with the number of receivers, K, and is independent of the number of antennas, N, which is much larger than K. In prior art of massive MIMO using the FDD mode, all N transmitters on the BS need to send pilots, separated in frequency or time, to the K receivers, which need to feedback the DL CSI to the BS. The overhead in sending the DL pilots scales linearly with N, the number of antennas on the BS, which can be a very large number, and the feedback of the DL CSI to the BS scales linearly with K, the number of receiving antennas. One prior art work [2] showed that for typical coherence block length, Multi-User MIMO (MU-MIMO) in FDD systems cannot afford a large number of BS antennas, otherwise the training and feedback overhead consumes the whole system throughput. Another prior work by the same authors [3] argued that the difficulty of using massive MIMO in FDD can be alleviated to some extent by Joint Spatial Division and Multiplexing (JSDM), which partitions the user population into groups that have similar transmitting correlation matrices and induce as small inter-group interference as possible by the user location geometry. However, these conditions may not be met and the overhead is still higher than in the TDD mode when they are met.
The challenge of CSI feedback in FDD networks is not limited to massive MU-MIMO. Coordinated Multi-Point transmission/reception (CoMP) in an FDD LTE network requires CSI estimation and feedback of channels in the CoMP measurement set, leading to much larger overhead than TDD network, especially for joint transmission and coordinated beamforming, because of lack of channel reciprocity.
This invention presents embodiments that solve the technical challenges discussed above for massive MIMO in FDD wireless communication.